Real lightsabers are closer than you think

WHEN it comes to making a blade of light, there are several insurmountable problems.

Light has a start point. But, to be a sabre, it also needs an end point.

We know light keeps on travelling at a steady, predictable rate - about 300,000km/s. Until it hits something. It's also invisible - unless beamed directly at you, or passing through a reflective cloud of dust or smoke.

And light doesn't repel light. There can be no guttural clash of blades: their beams would simply pass through each other.

So the awe-inspiring, glowing swords can't actually be made of light. Otherwise it would just be a torch. Or a laser.

But there is something it could be.

The young Luke Skywalker is introduced to a lightsabre by Jedi knight Obi-Wan Kenobi in Star Wars: A New Hope. Pucture: Lucas Film

WHAT IS A LIGHTSABER?

We know what it does. We've seen it often enough on the silver screen.

Switched on, it flashes out to form a bright, stuttering, humming blade. This has been shown to easily cut through structural pylons and bulkheads - as well as human and alien flesh.

It can also deflect blaster bolts, and other lightsabers.

"This can only be one thing, a plasma, somehow held in a sword-shaped region that also must be constrained by magnetic fields," says Swinburne University astrophysicist Associate Professor and Lead Scientist of Australia's Science Channel, Dr Alan Duffy.

"Why can I be so certain of this? Well because we already have constructed them."

What goes on inside one of these beasts produces an effect remarkably similar to the properties we observe with light sabres.

Plasma.

"Superheated atoms have their electrons ripped from the atomic nuclei leaving a charged, highly energetic plasma," he says.

"To have nuclear fusion occur, liberating energy in the process like the Sun, the temperatures of the plasma have to be enormous (over 100 million degrees). This high temperature plasma can easily damage the sides of any material you might hope to build the container from, and certainly can melt a bulkhead door."

The Tokamak reactor in dormant mode, left, and glowing as a magnetic field restrains its plasma. Picture: Supplied

The problem - in both the Star Wars universe and our own - is containing it.

Mere metals aren't up to the job.

"Fortunately, the plasma by its nature is charged," Dr Duffy says. "This means it can be trapped in a magnetic field."

This is why the Tokamak reactor is shaped a like a doughnut: it conforms to the flow of the magnetic fields that protect it from being slagged.

Take one Tokamak reactor. Miniaturise it. Manipulate the magnetic field.

"So if you have it constrained in a linear fashion then you have your sword," Dr Duffy says.

It can even look like a lightsabre. At least its cooler components can.

"The glow of the lightsaber is a little more intense than the ghostlike colour one sees in the Tokamak," he says, "but broadly speaking it's all sounding more science fact than science fiction."

The ability to ‘bounce’ lasers and plasma bolts is possible - even now - through atmospheric ionisation. Picture: Lucas Film

BOUNCING BLASTERS

We know light sabres can bounce blaster bolts.

But what are these anyway?

They're variously attributed to laser, plasma or particle beams.

Turns out, it doesn't really matter.

A Tokamak-style light sabre's intense magnetic field can deflect charged plasma blaster bolts. But not laser or particle beams.

Such directed energy weapons don't carry a charge.

But here, real physics offers a potential out.

"You'd have to form optical lenses from the air itself through the intense energy of the lightsaber that can then bend and deflect the light," Dr Duffy says.

Once again, it's something that's within the realms of possibility.

And we're already working on doing it.

Military technology researchers are studying the use of lasers to briefly ionise a patch of atmosphere into lens-shaped structures. These can magnify or distort the path of electromagnetic waves - including light.

The idea is to use it to jam enemy radio and radar signals - while boosting your own. It could also be used as a 'deflector shield' against attacks from laser weapons.

It's called a Laser Developed Atmospheric Lens.

Once again, says Dr Duffy, the issue with a light sabre is to miniaturise the effect sufficiently to be waved about in your hands.

Since you're already carrying about a piece of a star, there should be ample energy available.

But therein lies the catch.

Rey undergoes instruction in the use of her lightsaber in this promotional image for Star Wars: The Last Jedi.

POWER PLAY

"The challenge to replicate a Star Wars lightsaber is not the Force, it's the power," says Dr Duffy.

"Or rather lack thereof."

The experimental Tokamak reactor is yet to generate more power than what is sunk into it to create the plasma in the first place.

"The power it takes to run the Tokamak is enormous," Dr Duffy says. "Until they can liberate more energy from fusion than it takes to run them (known as Q-factor greater than one) the plasma in ITER will take 50MW to generate and contain."

What does this mean when scaled-down to a torch-like light sabre handle?

"The lightsabre is much smaller, typically 90cm in length and 8cm in diameter giving just over a litre volume of plasma, that means a millionth of the volume of ITER," he says.

"This means we only need 50W of power."

It's not a lot.

But have you ever seen a Jedi change his lightsaber's batteries?

A charge capable of operating for up to a year would need to contain half a MW.h (megawatt hour). This is the equivalent of roughly 200 Tesla PowerPacks

"So the Kyber crystals [the substance that powers the lightsabre, according to Star Wars lore] really pack in energy far more densely than even the best Lithium batteries do … roughly 50 million times more efficiently, in fact," Dr Duffy says.

The greatest challenge to building a lightsaber ... keeping all that harmful radiation away from the user.

THE HEAT IS ON

So can we build a light sabre?

No.

There remains several seemingly insurmountable hurdles to overcome before we can walk around with star blades in our hand.

The biggest is heat. And light.

Magnetic fields - while they can contain plasma - can't stop a blade from emitting radiation.

"At 100 million Kelvin, it's shining in the ultraviolet - almost soft X-rays - so you're in real danger of burning your own skin from energetic radiation from the lightsabre," Dr Duffy says.

"Perhaps this explains why long-term usage of the lightsaber by Sith lords causes them to look so ill. But their red lightsabers glow at a cooler temperature, and hence less dangerously energetic light, than bright blue Jedi lightsabers ..."

Under real physics, a light sabre must conform to what is known as the Stefan Boltzman equation:

The Stefan-Boltzmann equation calculates how much radiation escapes, and how far it reasches.

Here P is the amount of power radiated over an area, A. For perfect emitters of heat, that epsilon (a small variable) is 1.

This will all be shining at a temperature, T.

Sigma (the funny circle-squiggle) is the Stefan-Boltzmann constant - a figure that allows temperature to be converted to a measure of intensity.

That constant is:

The Stefan-Boltzmann constant: the total intensity radiated over all wavelengths increases as the temperature increases by a predictable degree.

"So … you really need to stop that heat loss. Somehow the lightsaber has to keep the plasma and light in," Dr Duffy says.

"In other words the field holding the plasma in place also has to be an almost perfect reflector of light too ... able to reflect almost all of the dangerous UV light that should be shining out at a good fraction of the Sun's entire output back into the blade".

An infinitesimally tiny amount of that light must still escape, however, so that the sabre appears to glow."

Such a field does not exist in our universe.

And when two magnetic fields collide - there can be fallout.

In the case of a star, it can form a solar flare.

When it comes to clashing light sabres, it could produce an explosive release of the plasma within each device.

So magnetic fields simply aren't viable - on their own.

Here's where science - as we know it - and fiction diverge.

Perhaps this is where "the Force" comes into the picture?

"A magnetic field can't do all that … and using 'the Force' doesn't cut it in our universe, I'm afraid," Dr Duffy says.